Method for forming thin composite solid electrolyte film for lithium batteries

ABSTRACT

A composite solid electrolyte film is formed by dissolving a lithium salt such as lithium iodide in a mixture of a first solvent which is a cosolvent for the lithium salt and a binder polymer such as polyethylene oxide and a second solvent which is a solvent for the binder polymer and has poor solubility for the lithium salt. Reinforcing filler such as alumina particles are then added to form a suspension followed by the slow addition of binder polymer. The binder polymer does not agglomerate the alumina particles. The suspension is cast into a uniform film.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected not to retain title.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 112,483 filedAug. 20, 1993, now issued as U.S. Pat. No. 5,360,686.

TECHNICAL FIELD

The present invention relates to elevated temperature, secondary lithiumbatteries and more particularly, this invention relates to a method offorming a composite, solid electrolyte film for such batteries.

BACKGROUND OF THE INVENTION

Advanced, high energy density batteries are required for use in space,military, communication and automotive applications. Certainjurisdictions, such as California, have mandated that an increasingpercentage of automobiles must be powered by electricity within the nextfew years. The lead-acid battery, though reliable and capable of manyrecharge cycles, is too heavy and has too low an energy to weight ratio.State of the art Ag--Zn and Ni--Cd batteries have poor charge retentionproperties and are also too heavy and bulky for use on space missionsand in some cases do not meet the life and environmental requirementsfor the missions.

Ambient temperature, secondary lithium batteries have several intrinsicand potential advantages including higher energy density, longer activeshelf life, and lower self discharge over conventional Ni--Cd, Pb--acidand Ag--Zn batteries. Successful development of these batteries willyield large pay-offs such as a 2-3 2 fold increase in energy storagecapability and a longer active shelf life of 2 to 4 years over Ni--Cd.These cells are most suitable for small spacecraft application requiringless than 1 kW power. Secondary lithium batteries are presently beingconsidered for a number of advanced planetary applications such as:planetary rovers (Mars Rover, Lunar Rover), planetary spacecraft/probes(MESUR, AIM, ACME Mercury Orbiter) and penetrators. These batteries mayalso be attractive for astronaut equipment, and Geo-SYN spacecraft.

Secondary lithium cells under development employ lithium metal orlithium ions in carbon as the anode, a chalcogenide salt such as TiS₂,MoS₂, MoS₃, NbSe₃, V₂ O₅, Li_(x) Mn₂ O₄, Li_(x) CoO₂, LiV₃ O₈ and Li_(x)NiO₂ as cathodes and liquid or solid electrolytes. During discharge ofthe cell, lithium metal is oxidized into lithium ions at the anode andlithium ions undergo an intercalation reaction at the cathode. Duringcharge, reverse processes occur at each electrode.

Solid polymer electrolyte/lithium batteries using polyethylene oxide(PEO) and other organic polymers complexed with lithium salts as theelectrolyte are under development. In addition, gel (polymer)electrolytes have received attention because of improved conductivityover the solid polymer electrolyte materials. These electrolytes havelow transference number (0.3-0.5) for lithium cations leading to highconcentration polarization and high interface resistance. Salt anions(BF₄ ⁻, AsF₆ ⁻, ClO₄ ⁻, CF₃ SO₃ ⁻) contained in the polymer are notcompatible with lithium and cause the lithium to degrade. The solidpolymer electrolytes (PE) have low mechanical strength especially above100° C. The disadvantages of prior art PE's deter development of highpower, high energy polymer lithium batteries for the following reasons.In the PE's the Li cation, which is complexed (bound) to the polymer haslow mobility, while the uncomplexed anion moves faster. The activity ofthe salt anions with lithium results in a thick lithium passivatinglayer which has high resistance. Also, above 100° C. the prior art PE'sbecome soft and start to flow.

These problems were addressed by changing the mechanism for conductionof lithium ions, eliminating the non-compatible ions and usingcompatible ions such as halide and adding an inorganic filler as areinforcing agent.

Addition of Al₂ O₃ (2) (3) has improved the mechanical strength ofpolymer electrolytes. However, the lithium transference number was lowbecause the salt was not compatible with lithium ions, the saltconcentration was too low and the Al₂ O₃ particles were too large. Solidlithium iodide (LiI) has good ionic conductivity and low electronicconductivity. Its lithium transference number is close to unity.Conduction is accomplished through a lithium vacancy mechanism (1). Itwas found that by mixing LiI and Al₂ O₃ powders and pressing them into apellet an order of magnitude is gained in conductivity over pure LiI.This is due to the presence of Al³⁺ cations at the LiI interface whichresults in an increase in the Li⁺ vacancy concentration. The Li⁺conduction is carried out in the LiI mainly at the LiI/Al₂ O₃ interface.However, a LiI-Al₂ O₃ pressed pellet is very brittle and has poormechanical and shock properties. In practice thick pellets are requiredto avoid these problems. This principle was the basis for the solidstate batteries used in low rate medical applications.

LIST OF CITED REFERENCES

1. Comprehensive Treatise of Electrochemistry, Vol. 3 (1981) Bockris etal. Editors

2. J. E. Weston and B. C. H. Steele, Solid State Ionics 7, 75-79 (1982)

3. F. Croce, F. Bonino, S. Panero, and B. Scrosati, PhilosphicalMagazine 59 161-168 (1989)

STATEMENT OF THE INVENTION

An improved composite solid electrolyte (CSE) containing LiI and Al₂ O₃is disclosed in copending application entitled THIN COMPOSITE SOLIDELECTROLYTE FILM FOR LITHIUM BATTERIES Filed Aug. 20, 1993, as Ser. No.08/112,483, now issued as U.S. Pat. No. 5,360,686, the disclosure ofwhich is expressly incorporated herein by reference. The improved CSEhas mechanical properties superior to that of PEO/LiI and exhibits a Li⁺transport number close to one which has never before been reported asachieved in a CSE. This completely eliminates concentration andpolarization effects and enhances high rate and power capability. Thesalt anion (I⁻) is totally compatible with lithium resulting in a stablesystem. Because of total compatibility and because of an improvedlithium transport mechanism, the interfacial Li/CSE resistance is lowerby as much as a factor of 10 when Compared with the prior art polymerelectrolytes (PE) described above. The ionic conductivity of the CSE at20°-120° C. is similar to or better than the prior art PE's. The novelCSE will allow development of lithium/TiS₂ (or other cathode) batterieswith power densities above 100 Wh/Kg (pulse power capability of 1 Kw/Kg)and specific energy of 100 Wh/Kg (based on full battery).

The conduction mechanism for the improved CSE is completely differentfrom that occurring in polymer electrolytes. The Li⁺ conduction isprimarily carried out in an inorganic solid matrix rather than in a saltloaded organic polymer or gel matrix. The improved CSE contains onlyanions such as halide compatible with Li⁺. The Al₂ O₃ inorganic fillerstrengthens the CSE film and eliminates flow of the film at hightemperature.

In the improved CSE, compatible lithium halide is coated as a thin filmonto the surface of the reinforcing particles. The particles are thenbonded together with a polymer which can be a polyelectrolyte such asPEO. The LiI retains the vacancy conduction mechanism for Li⁺ which isresponsible for the transference number near unity for lithium. Thebinder polymer retains its flexibility. The polymer can function solelyas a binder with all conduction occurring in the solid LiI coatedparticles or if it is a polyelectrolyte, it can serve as a solidelectrolyte providing ionic conductivity between the solid particlesdispersed in the polymer and bound together by the polymer.

However, when the CSE film was cast from a mixture of polymer such aspolyethylene oxide, alumina and lithium iodide from a polar solvent suchas acetonitrile, the alumina tended to agglomerate into small ballscovered with the polymer. It appears that PEO acts as a scavenger. Theagglomerated film was not uniform in composition or thickness. Onlysmall areas of the order of a few millimeters were uniform incomposition.

STATEMENT OF THE INVENTION

A method of forming large area, uniform, composite, solid electrolytefilms is provided by the invention. The film is uniform over areas atleast as large as 5 centimeters permitting formation of films largeenough to be useful as a solid electrolyte in a lithium battery.

In the method of the invention, a second solvent which is a solvent forthe binder polymer and which has poor solubility for the lithium salt atroom temperature is added to the first solvent. The first solvent is acosolvent for the binder polymer and the lithium salt. The binderpolymer is believed to partition between the two solvents preventing thepolymer from agglomerating the alumina filler particles as the film setsduring casting.

The first solvent is preferably an aprotic organic solvent such asacetonitrile or propylene carbonate. The second solvent can be a loweralkanol containing 1 to 10 carbon atoms, preferably a branched chainalkanol containing 3-6 carbon atoms such as isopropyl alcohol. The twosolvents are utilized in about equal amounts by volume usually from 40%to 60% by volume of isopropyl alcohol (IPA), remainder beingacetonitrile (MeCN).

The two solvents can be added stepwise to the solution. The lithium salt(LiI) is first added to the first solvent (MeCN). The solution isdecanted. The filler (alumina) is added with stirring from 15 to 60minutes to form a suspension. A first portion of the second solvent(IPA) is added and stirred well. The second portion of the first andsecond solvents are then added. The binder polymer (PEO) is then addedslowly with vigorous stirring. A uniform suspension results. On castingof the suspension into films having a thickness from 100 μm to 200 μm,uniform CSE films were produced.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

DISCUSSION OF THE PRIOR ART

U.S. Pat. No. 5,204,196 discloses a solid state conductive polymercomposition formed a host copolymer including polyethylene oxide. Theelectrolyte can include lithium salt.

U.S. Pat. No. 5,154,991 discloses a flexible, electrolyte separatorformed by mixing alumina, teflon and an isopropanol solution in water,roll pressing and sintering.

U.S. Pat. No. 5,057,360 discloses a ceramic precursor including aluminaand a polymer cast from a slurry in which the solvent can beisopropanol.

U.S. Pat. No. 5,112,512 discloses a solid polymer electrolyte formedfrom a polyoxyalkalene copolymer and an organopolysiloxane, and whichcan include lithium iodide.

U.S. Pat. No. 4,990,413 discloses a composite solid electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a NyQuist plot of a composite solid electrolyte of theinvention containing 0.05 micron particles;

FIG. 1B is a NyQuist plot of a composite solid electrolyte of theinvention containing 0.30 micron particles;

FIG. 2 is a plot of conductivity vs. 1/T of the composite solidelectrolyte;

FIG. 3 is a plot Creep % vs. Time of PEO/LiI with and without Al₂ O₃

FIG. 4 are plots of D-C cyclic voltammetric characteristics of aLi/CSE/TiS₂ cell;

FIG. 5A is a plot of a Diffusion coefficient;

FIG. 5B is a plot of R_(ct) vs. OCV;

FIG. 6 is a plot showing charge/discharge characteristics of aLi/CSE/TiS₂ cell; and

FIG. 7 is a schematic representation of a thin film batteryincorporating the composite solid electrolyte of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the composite solid electrolyte (CSE) film 10 of theinvention is utilized in a thin, solid state battery. A battery 12includes an anode 14 comprising a thin film of lithium metal or an alloythereof, the composite solid electrolyte film 10 and a cathode 16 whichmay be attached to a current collector 18 such as stainless steel. Thebattery may be enclosed in a hermetically sealed polymeric enclosure 20.Leads 22, 24 connect the anode 14 and cathode 16 to terminals 26, 28.The battery may suitably be provided in disc form with a terminal 26, 28on the top and bottom faces, respectively.

The composite solid, polymeric electrolyte can be a non-conductivehydrocarbon polymer such as polyethylene or polypropylene or aconductive polymer, suitably a polyelectrolyte such as a polyalkyleneoxide or a polyacrylonitrile. Polyethylene oxide polymers having amolecular weight from about 10⁴ to about 10⁷, suitably, about 10⁶ can beused to form the CSE. Other polymers may optionally be present. From 0to 30% by weight of polymers such as polypropylene, polyphosphazene,polysiloxane, polyurethane or polyethylene may be mixed with thepolyethylene oxide in order to modify the physical and electrochemicalperformance of the solid polyelectrolyte. The solid polyelectrolytecontains, based on the oxygen content of the polyethylene oxide, from1/1 to 100/1 of a lithium ion provided as a compatible salt such as alithium halide. The lithium salt is deposited on small sized, inert,inorganic particles such as Al₂ O₃. Usually the ratio of O/Li is from1/1 to 10/1. The lithium salt is preferably lithium iodide.

The composite solid polyelectrolyte film nominally contains from 4-20percent by weight of inorganic filler particles, preferably from 6-15%filler, from 15-40% by weight of a compatible lithium salt coated ontothe particles and 0 to 10% by volume of plasticizer such as Triglyme.

A CSE was prepared by dissolving the lithium halide in a solvent such asacetonitrile and decanted. Alumina is added to the solution and thesolution is stirred well. A further quantity of solvent was addedfollowed by the slow addition of a binder resin. It was found that inthe case of a polyelectrolyte such as PEO, the PEO immediatelyagglomerates the alumina into small balls covered with PEO. The PEOappears to act as a scavenger. The composition was not uniform.

In a modified procedure, isopropyl alcohol is added to the acetonitrilesolution containing the lithium salt before the addition of PEO. Thisprovides a uniform suspension of alumina coated with the lithium salt.

EXAMPLE FOLLOWS

Amounts of LiI, Al₂ O₃ and PEO (M.W.4×10⁶) as shown in the followingtable were separately weighed.

    ______________________________________                                                  Al.sub.2 O.sub.3,          Triglyme,                                EXAMPLE   wt %     LiI, wt % PEO, wt %                                                                             wt %                                     ______________________________________                                        1         10       35        55      0                                        2         6. 4     23        63      7                                        ______________________________________                                    

LiI was dissolved in 50 ml of acetonitrile and the solution decanted.Alumina (both 0.05 micron and 0.30 micron) was added to the solutionwith stirring for 45 minutes. 80 ml of isopropyl alcohol (IPA) wasadded. The solution was again stirred well. 120 ml of acetonitrile wasadded to this suspension followed by another 80 ml of IPA. 1.6 grams ofPEO (M.W.4×10⁶) was added slowly while the solution was being vigorouslystirred. A uniform suspension of lithium iodide coated alumina particleswas produced. The mixture was stirred overnight to dissolve the PEO. Thesuspension was then cast into films.

Thin films of CSE prepared by the modified procedure were subjected to aseries of electrochemical measurements including a-c and d-cmeasurements. Both a symmetrical cell of the type Li/CSE/Li and anunsymmetrical cell of the type Li/CSE/SS (stainless steel) were used forthe electrochemical characterization of the CSE films. With TiS₂ as thecathode a small capacity cell was fabricated and charge/dischargestudies were made.

A further discovery of the invention is the influence of the size of theinorganic particle on the electrical performance of the electrolytefilm. It has been found that electrical performance of the CSE issignificantly higher when the filler particles are below 0.5 micron insize, preferably from 0.01 to 0.1 micron in size.

RESULTS AND DISCUSSION

A) BULK CONDUCTIVITY AND INTERFACIAL CHARGE TRANSFER RESISTANCE:

(Both the bulk conductivity (1/R_(b)) and the interfacial chargetransfer resistance (R_(ct)) of the electrolyte (CSE) were determinedfrom the a-c measurements. The a-c measurements were made in thefrequency regime 100 KHz-5 Hz. A typical NyQuist plot is shown in FIGS.1A and 1B for CSE films containing 0.05 and 0.3 micron alumina,respectively. While the high frequency intercept on the x-axis is thebulk resistance of the electrolyte the corresponding low frequencyintercept gives the combination of the bulk resistance of theinterfacial layer (present on the Li surface) and the charge transferresistance, which was defined earlier as Rot. The CSE film containing0.3 micron Al₂ O₃ (FIG. 1B) exhibits three different regimes dominatedby bulk processes at high frequencies followed by charge transferprocesses at medium frequencies which in turn is followed by diffusionalprocesses at low frequencies. However, the CSE films with 0.05 micronAl₂ O₃ (FIG. 1A) exhibits almost resistor like behavior where thecontribution from the charge transfer and diffusional processes areinsignificant. The a-c characteristics of the CSE films with 0.3 micronAl₂ O₃ is typical of systems where the transport number of thereversible ion is very low. The behavior of CSE films with 0.01 micronAl₂ O₃ is similar to that of 0.05 micron Al₂ O₃ film.

In FIG. 2 is a plot of the bulk conductivity of a CSE containing 0.05micron of Al₂ O₃ as a function of the reciprocal temperature. The dataindicate that while the CSE exhibits a very modest conductivity below79° C., above this temperature the conductivity picks up. Further thetemperature (79° C.) at which the break occurs is higher than for PEOwithout the alumina. For PEO systems without alumina, the break inconductivity occurs around 60° C. The interfacial charge transferresistance appears to be stable over a period of many days.

B) TRANSPORT NUMBER

The transport numbers of the cation and anion represent the ratio of thetotal current that will be carried by the cations and anionsrespectively. The cation transport number is close to unity. In Table 1,electrochemical data are compared with the data available in theliterature for comparable systems. The data indicate that not only isthe transport number higher but the R_(ct) is lower for our systemcompared to state-of-the-art CSE systems.

                  TABLE 1                                                         ______________________________________                                                            Film bulk         Interface                                           Temp    Cond. mho         resistance                              Mat. Comp.  °C.                                                                            cm.sup.-  tLi.sup.+                                                                             ohm cm.sup.2                            ______________________________________                                        (LiI).sub.1 (PEO).sub.3                                                                   116     6 × 10.sup.-4                                                                     0. 8 ± 0.05                                                                        2.5                                     (Al.sub.2 O.sub.3).sub.0.3                                                                 90     2 × 10.sup.-4                                                                     0.9 ± 0.05                                                                         10                                      (LiI).sub.1 (PEO).sub.165                                                                 103     10.sup.-4 1 ± 0.05                                                                           25                                      (Al.sub.2 O.sub.3).sub.0.39                                                   PRIOR ART                                                                     (PEO).sub.8 NaI                                                                           120     3 × 10.sup.-4                                       10% Al.sub.2 O.sub.3 (3)                                                      (PEO).sub.8 LiClO                                                                         118     10.sup.-3 0.22    25                                      10% Al.sub.2 O.sub.3 (2)                                                      (PEO).sub.4.5 LiSCN                                                                       115     10.sup.-4 0.5     72                                      (6a)                                                                          ______________________________________                                    

In FIG. 3 a typical plot of creep % as a function of time is shown fortwo different polymer electrolytes, one containing alumina (CSE) and theother without. The results indicate that the CSE of the invention ismuch more dimensionally stable than the PEO/LiI electrolyte.

D) STUDIES ON Li/CSE/TiS₂ CELLS

A 10 mAh small capacity cell was made with TiS₂ as a cathode and d-ccyclic voltammetric measurements were made as a function of open circuitvoltages (OCVS). In FIG. 4 is shown a typical d-c cyclic plot and in thesame figure is shown the peak splitting as a function of OCV. The welldefined cathodic and anodic peaks indicate that Li⁺ moves in and out ofthe TiS₂ cathode (the cell can be charged and discharged). The peaksplitting increases with decrease in OCV of the cell which may berelated to the increase in resistance of TiS₂ with lithiation. In FIG.5A is shown the plot of diffusion coefficient of Li⁺ in TiS₂ as afunction of OCV and FIG. 5B shows charge transfer resistance at the TiS₂electrode also as a function of OCV. While the R_(ct) varies randomlywith OCV the diffusion coefficient goes through a maximum at around 50%state-of-charge. A similar observation was made earlier for TiS₂ cathodewith organic electrolytes. In FIG. 6 is shown the charge/dischargecharacteristics of the above cell. The cell was discharged at C/20 andcharged at C/40 rates. Although the transport number for Li is close tounity the charge/discharge rates are very low. One explanation would bethat the CSE bulk ionic conductivity is still very low by an order ofmagnitude than the required minimum of 10⁻³ S cm⁻¹.

CONCLUSIONS

The method of the invention permits formation of uniform films of CSEwith excellent mechanical properties. The composite solid electrolyte(CSE) prepared by the method of the invention exhibits the highesttransport number reported yet for a polymeric electrolyte for Li⁺. Theconductivity of the CSE at 103° C. is 10⁻⁴ mho cm⁻¹. Both the transportnumber and ionic conductivity are influenced by the particle size ofalumina. Thermal creep measurement studies show that the CSE is muchmore dimensionally stable than the PEO/LiI electrolyte.

It is to be realized that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

We claim:
 1. A method of forming a film of a composite solid electrolytecomprising the steps of:dissolving a lithium salt in a first solvent toform a solution, the first solvent being a cosolvent for the lithiumsalt and a binder polymer; adding particles of a reinforcing filler tothe solution to form a suspension; adding a second solvent to thesuspension, the second solvent being a solvent for the binder polymerand having low solubility for the lithium salt; then adding the binderpolymer to the suspension with stirring to dissolve the binder polymerin the suspension and form a uniform suspension of lithium salt coatedparticles of reinforcing filler in said solution of binder polymer; andcasting the uniform suspension into a film.
 2. A method according toclaim 1 in which the binder polymer is a polyelectrolyte polymer.
 3. Amethod according to claim 2 in which the polyelectrolyte comprises apolyalkylene oxide.
 4. A method according to claim 2 in which thepolyelectrolyte comprises a polyethylene oxide.
 5. A method according toclaim 3 in which the particles of filler are a refractory material.
 6. Amethod according to claim 1 in which the particles comprise alumina. 7.A method according to claim 5 in which the lithium salt is a lithiumhalide.
 8. A method according to claim 7 in which the lithium halide isa lithium iodide.
 9. A method according to claim 7 in which the firstsolvent is an aprotic solvent.
 10. A method according to claim 9 inwhich the second solvent comprises an alkanol containing 1 to 10 carbonatoms.
 11. A method according to claim 10 in which the alkanol containsa branched chain.
 12. A method according to claim 11 in which thealkanol is a secondary alkanol.
 13. A method according to claim 12 inwhich the alkanol is isopropyl alcohol.